TM 55-1510-219-10 ground, turn the bleed air valve switches to the ENVIRO OFF position. (2) The forward vent blower is controlled by a switch placarded VENT BLOWER AUTO LOW HI.  The HI and LOW positions regulate the blower to two speeds of  operation.    IN  the  AUTO  position,  the  fan  will  run  at low speed except when the CABIN TEMP mode selector switch   is   placed   in   the   OFF   position.      In   the   OFF position, the blower will not operate. (3) The   aft   vent   blower   is   controlled   by   a switch placarded AFT VENT BLOWER OFF AUTO ON. In the OFF position, the blower will not operate. Section IX.  ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEM 2-71.  DESCRIPTION. The aircraft employs both direct current (DC) and alternating   current   (AC)   electrical   power.      The   DC electrical  supply  is  the  basic  power  system  energizing most  aircraft  circuits.    Electrical  power  is  used  to  start the engines, to power the landing gear and flap motors, and   to   operate   the   standby   fuel   pumps,   ventilation blower,  lights  and  electronic  equipment.    AC  power  is obtained  from  DC  power  through  inverters.    The  three sources    of    DC    power    consist    of    one    20    cell    34- ampere/hour     battery     and     two     400ampere     starter- generators.    DC  power  may  be  applied  to  the  aircraft through    an    external    power    receptacle    on    the    right nacelle.        The    starter-generators    are    controlled    by generator  control  units.    The  output  of  each  generator passes through a cable to the respective generator bus (fig.    2-22).    Other  buses  distribute  por  to  aircraft  DC loads, and derive power from the generator buses.  The generators   are   paralleled   to   balance   the   DC   loads between  the  two  units.    When  one  of  the  generators  is not on line, aircraft DC power requirements continue to be supplied from one of the other generators.  Most DC distribution    buses    are    connected    to    both    generator buses    but    have    isolation    diodes    to    prevent    power crossfeed     between     the     generating     systems,     when connection  between  the  generator  buses  is  lost.    Thus, when either generator is lost because of a ground fault, the operating generator will supply power for all aircraft DC    loads    except    those    receiving    power    from    the inoperative  generator's  bus  which  cannot  be  crossfed. When a generator is not operating, reverse current and over-voltage  protection  is  automatically  provided.    Two inverters operating from DC power produce the required single-phase  AC  power  (figure  2-23).    Three  phase  AC electrical    power    for    inertial    navigation    system    and mission    avionics    is    supplied    by    two    DC    powered inverters (figures 2-24 and 2-25). 2-72.  DC POWER SUPPLY. One  nickel-cadmium  battery  furnishes  DC  power when  the  engines  are  not  operating.    This  24-volt,  34- ampere/hour  battery,  located  in  the  right  wing  center section, is accessible through a panel on the  2-58 top of the wing.  DC power is produced by two engine-driven 28 volt, 400-ampere starter-generators.  Controls and indicators   associated   with   the   DC   supply   system   are located  on  the  overhead  control  panel  (fig.    2-18)  and consists of a single battery switch (BATT), two generator switches    (#1    GEN    and    #2    GEN),    and    two    volt- loadmeters. a. Battery  Switch.    A  switch,  placarded  BATT  is located on the overhead control panel (fig.  2-18) under the  MASTER  SWITCH.    The  BATT  switch  controls  DC power   to   the   aircraft   bus   system   through   the   battery relay, and must be ON to allow external power to enter aircraft circuits.  When the MASTER SWITCH is moved aft, the BATT switch is forced OFF. NOTE With    battery    or    external    power removed from the aircraft electrical    system,    due    to    fault, power  cannot  be  restored  to  the system  until  the  BATT  switch  is moved to OFF/RESET, then ON. b. Generator  Switches.    Two  switches  (fig.    218), placarded   #1   GEN   and   #2   GEN   are   located   on   the overhead  control  panel  under  the  MASTER  SWITCH. The  toggle  switches  control  electrical  power  from  the designated  generator  to  paralleling  circuits  and  the  bus distribution   system.      Switch   positions   are   placarded RESET,   ON   and   OFF.      RESET   is   forward   (spring- loaded   back   to   ON),   ON   is   center,   and   OFF   is   aft. When a generator is removed from the aircraft electrical system,  due  either  to  fault  or  from  placing  the  GEN switch in the OFF position, the affected unit cannot have its output restored to aircraft use until the GEN switch is moved to RESET, then ON. c. Master  Switch.    All  electrical  current  may  be shut off using the MASTER SWITCH gangbar (fig. 2-18)  which  extends  below  the  battery  and  generator switches.    The  MASTER  SWITCH  gangbar  is  moved forward when a battery or generator switch is turned on. When moved aft, the bar forces each switch to the OFF position. 2-58